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Author Topic: Dentron MLA-2500 & GI-7B Tubes  (Read 1057 times)
AH6RR
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« on: April 17, 2006, 09:55:13 PM »

Aloha,
 I have just converted my MLA-2500 to use the Russian GI-7B tubes but I am having a problem getting the bias right. I followed the W4EMF mod but I am confused on some of his Bias circuit hook up. Doing the filiment center tap to the K8RA amplifier operational Bias module using a 37 volt zenner insted of the 7.5 v .
1. Do I use the old zener wire for the return path to B- or do I hook it into another part of the B- path? ( no matter how I configure it I cannot get the resting current to get below 175 mils )
2. I also added Rich Measures AG6K mods for the ALC  circuit and the parasitic suppressors.
 
Plate voltage is 2250V and there is no problem in the high voltage circuit but with the high resting current of 175 mils the tubes get hot very fast. I have already destroyed 1 tube already thank God they are only 24 bucks each. Output before the tube poped was 1kw with 70 watts of drive.
 
Any Help with the bias would be most helpful
 
Thanks
Roland AH6RR
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KA5N
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Posts: 4380




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« Reply #1 on: April 18, 2006, 06:02:59 AM »

In another W4EMR conversion he uses a 41 volt zener to get 100 ma resting current.  He also mentions that - 24-30 volts bias is required.  I think you must increase the bias voltage to get the resting current lower.  Probably somewhere around 41-50 volts zener.  You will also need to insert a standby resistor in the bias circuit (if you don't already have it).  As far as the "original zener lead" I am not sure what you are asking.
GL
Allen
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WIRELESS
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« Reply #2 on: April 18, 2006, 06:19:52 AM »

What do you mean by reating current?  Current when unkeyed?  Current when keyed but not driven with RF?  Unkeyed current should be zero (by raising bias). Keyed current doesn't sound that high for AB1 amp but I am not that familiar with your tube.  Many AB1 tubes run very close to max plate diss. when just keyed.  How much air do you have on the tube and are plate seals below max?  AB1 amps require better cooling than zero bias triode amps since efficiency is better with AB2 or B.

Tetrode amps can be very confusing if built by those that don't have a lot experience with them.  You might want to find someone locally that can help you if they have past experience with tetrode or penthode rf amps.

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WIRELESS
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« Reply #3 on: April 18, 2006, 06:28:20 AM »

After doing some searching I learned that your tube is a triode.  This tube looks like it is some sort of special purpose tube with various plate cooling options.  Since this is a relatively rare tube, good luck finding help.  I never heard of it, myself.  If I was going to convert a 8875 tube amp I think a 3cx800a7 or 8874 would have been a better but more expensive choice.  Good Luck.
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AH6RR
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« Reply #4 on: April 18, 2006, 08:21:07 AM »

The resting current with no rf is 175 mils I do not have a rig hooked up to it. I am using a 37 volt zener. It make no differnce if the zener is os is not hooked up in the old zener circuit. I guess I need to find a differnt path to B- and meters to install the zener. The reason I chose the GI-7B is cost as the 3cx400, 8874 tubes are over $200 each and the russian tube is $25 each I'm on a tight buget here.
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WIRELESS
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« Reply #5 on: April 18, 2006, 01:30:48 PM »

Trying to figure this out over the internet is probably futile but I suggest that you find some GG amp circuits in a hand book and try to compare it to your circuit and see if you can find an obvious major difference.  All GG amps are very similiar.  A bias of around 10 volts is usually enough for correct idling current.  Bias of 25+ volts may be enough to cut off plate current when unkeyed.  The idle current of 175 mils for 2 tubes doesn't sound unusual for an undriven but keyed amp.  Unkeyed, the current should be near zero.  Good luck.
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AH6RR
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« Reply #6 on: May 07, 2006, 09:13:17 AM »

Thanks for all the Help. I found the problem and it is now working just fine with 1100 watts output on all bands. It was my fault I was reading too much into the mod. I just put it back to stock changed a 47K resistor to 100K and changed the 50 watt zener 7.2V to 25V. I got some very good reports on 20 meters last night. I'm a happy camper now :=)

Roland AH6RR
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